Human Anatomy - Kotsan I. Ya. 2009

Neural Pathways of the Brain and Spinal Cord
Pyramidal Tracts

The pyramidal tracts are responsible for the conscious (voluntary) control of skeletal musculature and the execution of highly differentiated, precise movements. These tracts are subdivided into the corticospinal and corticonuclear (corticobulbar) pathways.

The key Features of the pyramidal tracts include:

1) the first Neurons are represented by large pyramidal Cells, including the giant Betz cells, whose Cell bodies are located in layer V of the Cerebral Cortex (primarily the precentral gyrus, etc.);

2) within the cerebral hemispheres, the fibers of the pyramidal tracts course through the corona radiata and the internal capsule;

3) in the Brainstem, the fibers are situated in its ventral portion, passing sequentially from the cerebral peduncles into the Pons and Medulla Oblongata;

4) in the caudal region of the medulla oblongata, at the border with the Spinal Cord, 80% of the fibers cross to the opposite side, forming the pyramidal decussation (motor or lower decussation);

5) within the spinal cord, the pyramidal tract fibers travel in the lateral funiculi (lateral corticospinal tract) and anterior funiculi (anterior corticospinal tract);

6) The Cell bodies of the second neurons are located in the motor nuclei of the Cranial Nerves (corticonuclear pathway) or the motor nuclei of the anterior horns of the spinal cord (corticospinal pathway);

7) the axons of the second neurons travel within the cranial or Spinal Nerves to the skeletal musculature, where they terminate in effectors.

The corticospinal (pyramidal) tract (tractus corticospinalis pyramidalis) conducts nerve impulses from the pyramidal cells of layer V of the cerebral cortex to the neurons of the motor nuclei in the anterior horns of the spinal cord.

Approximately 30% of the fibers of the pyramidal tract originate in the precentral gyrus, and 20% in the cortex of the postcentral gyrus; additionally, the pyramidal bundle arises from the posterior third of the superior parietal lobule and the supramarginal gyrus of the cerebral hemisphere.

The first neurons are represented by the giant Betz cells and large pyramidal cells; however, fiber counts of the pyramidal tract have shown that large pyramidal cells outnumber Betz cells tenfold.

The axons of the first neurons of the pyramidal tract enter the corona radiata and descend, intersecting the fibers of the corpus callosum along their course. They then form a compact bundle passing through the internal capsule between the thalamus and the lentiform Nucleus. Here, the corticonuclear fibers occupy a medial position within the genu, while the corticospinal fibers occupy the anterior two-thirds of the posterior limb of the internal capsule.

Further down, the fibers of the corticospinal tract pass through the middle of the Base of the ventral cerebral peduncle and the anterior part of the pons. Within the pons, the pyramidal tract fibers lose their compactness, breaking up into smaller fascicles intermingled with the fibers of the pontocerebellar tract and numerous intrinsic pontine nuclei.

In the medulla oblongata, the pyramidal tract fibers converge once again to form the pyramids, which appear on its ventral surface as two longitudinal bulges separated by the anterior median fissure.

In the lower part of the medulla oblongata, near the border with the spinal cord, the pyramidal tract divides into two bundles: the major portion (80%) crosses over to the opposite side, descending in the lateral funiculi of the spinal cord after the pyramidal decussation to form the lateral corticospinal tract. The remaining smaller portion (20%), which does not participate in the decussation, stays on its own side and travels down the anterior funiculi to form the anterior (direct) corticospinal tract.

The lateral corticospinal tract occupies the posteromedial part of the lateral funiculus of the spinal cord, gradually tapering in the caudal direction.

As they descend through the anterior funiculi of the spinal cord, the fibers of the anterior corticospinal tract successively cross the white commissure to the opposite side, where they terminate on the motor neurons of the anterior horns of the spinal cord.

Thus, all fibers of both the anterior (direct) and lateral corticospinal tracts ultimately become crossed. It has been established that only 20% of these fibers synapse directly onto the motor neurons of the anterior horns of the spinal cord, whereas the majority (80%) connect with motor neurons via interneurons. Because the pyramidal tract fibers give off numerous collaterals along their course, nerve impulses from each individual fiber simultaneously reach neurons across several spinal segments. Specifically, 55% of all pyramidal tract fibers terminate in the cervical segments of the spinal cord, 20% in the thoracic segments, and only 25% in the lumbar segments. The fibers of the anterior corticospinal tract do not descend below the thoracic segments.

The axons of the second neurons (motor neurons) emerge from the spinal cord as part of the anterior roots and then travel within the spinal nerves to the skeletal Muscles of the Trunk, neck, and limbs. The impulses discharged by these motor neurons trigger Muscle contraction. Thanks to feedback loops, a stream of impulses from the muscles is relayed via dedicated pathways back to the same motor neurons, signaling the Current state of the muscles (the degree of their contraction). This enables the motor neurons to appropriately enhance or diminish muscle tension depending on the specific situation.

Three distinct types of cells are distinguished among the motor neurons of the anterior horns of the spinal cord:

1. Large alpha cells with thick axons, which drive rapid muscle contraction because such thick axons conduct impulses at a very high velocity.

2. Small alpha cells with thinner axons, whose function is related to the maintenance of muscle tone.

3. Gamma cells featuring very thin axons that conduct nerve impulses relatively slowly.

The pyramidal tract consists of myelinated and Cytology/practical/64.html">Unmyelinated nerve fibers ranging from 1 to 8 µm in diameter. The maximum conduction velocity of nerve impulses along the fibers of the pyramidal tract does not exceed 60–65 m/s.

Since the fibers of the anterior and lateral pyramidal tracts cross, the innervation of muscles is contralateral. The right hemisphere controls the left half of the body and vice versa. The upper part of the precentral gyrus and paracentral lobule is associated with the Muscles of the Lower Limb, the middle part of the gyrus with the muscles of the trunk and upper limb, and the lower part with the muscles of the neck and HEAD (face, Tongue, Pharynx, Larynx).

Complete damage to the pyramidal tract within the Brain AND SPINAL cord, extending from the cortical pyramidal neurons to the motor neurons of the anterior horn of the spinal cord, leads to central paralysis. When the integrity of the axon or cell body of a spinal motor neuron is disrupted, peripheral paralysis occurs.

A lesion of the pyramidal tract at the level of the spinal cord above the cervical enlargement results in paralysis of both the upper and lower limbs on the ipsilateral side. If the pathological focus is located below the cervical enlargement, paralysis occurs only in the lower limb on the ipsilateral side.

The corticonuclear tract (tractus corticonuclearis) is a two-neuron, descending, motor pathway. It is an integral component of the pyramidal system and serves for the conscious control of striated muscles of the head and, partially, the neck, which are innervated by the cranial nerves (III, IV, V, VI, VII, IX, X, XI, XII pairs). Movements performed via this pathway are characterized by exceptional precision and fine differentiation.

The corticonuclear tract originates from large pyramidal neurons whose cell bodies lie in layer V of the cortex in the lower third of the precentral gyrus. Their axons descend as part of the corona radiata, then pass through the genu of the internal capsule, The basis of the middle part of the cerebral peduncle, the basilar pons, and reach the central part of the medulla oblongata.

Along their course, some fibers of the corticonuclear tract gradually cross to the opposite side, terminating near the Cells of the motor nuclei of the cranial nerves: oculomotor (III) and trochlear (IV) in the Midbrain; trigeminal (V), abducens (VI), and facial (VII) in the pons; glossopharyngeal (IX), vagus (X), accessory (XI), and hypoglossal (XII) in the medulla oblongata. The remaining fibers reach the ipsilateral nuclei on their own side.

Within the pyramidal bundle, the corticonuclear fibers occupy a medial position.

The axons of the second-order neurons, whose cell bodies form the motor nuclei of the cranial nerves, run within the respective cranial nerves to the skeletal muscles of the Head and Neck.

As part of the III, IV, and VI pairs, they supply the extraocular muscles:

III pair: superior, medial, and inferior rectus, and inferior oblique muscles, as well as the levator palpebrae superioris muscle;

IV pair: superior oblique muscle of the eye;

VI pair: lateral rectus muscle of the eye.

As part of the remaining nerves:

V pair: medial and lateral pterygoid and masticatory muscles, mylohyoid muscle, anterior belly of the digastric muscle, and the tensor veli palatini and tensor tympani muscles;

VII pair: Muscles of facial expression, stapedius muscle, platysma, posterior belly of the digastric muscle, and stylohyoid muscle;

IX pair: stylopharyngeus muscle;

X pair: striated muscles of the pharynx, soft palate, larynx, and upper Esophagus;

XI pair: trapezius and sternocleidomastoid muscles;

XII pair: muscles of the tongue (superior and inferior longitudinal, genioglossus, styloglossus, hyoglossus) and, via the ansa cervicalis, the Infrahyoid muscles (sternohyoid, sternothyroid, thyrohyoid, and omohyoid).

Unilateral damage to pyramidal cells or any section of the corticonuclear fibers most commonly causes paresis rather than paralysis (limitation of voluntary movements, decrease in muscle contractile strength), because the cells of the cranial nerve motor nuclei receive impulses from cortical centers of both hemispheres due to the partial decussation of pathways.

Bilateral damage to the corticonuclear tracts causes central paralysis.